It may be remembered as the moment that the GOP establishment's long-brewing horror over the billionaire businessman burst into open political combat and an active bid to bring him down at the party convention.

"I believe with all my heart and soul that we face another time for choosing, one that will have profound consequences for the Republican Party and more importantly, for the country," said Romney, channeling former President Ronald Reagan in his stunning speech in Utah.

Read More

Romney's concise, categorical takedown of Trump's intellect, character and motivation amounted to a tipping point in a long-building revolt among Republican elders now openly despairing of the former reality TV star's grip on the GOP nomination and his staunch armies of outsider voters who refuse to abandon their outspoken champion.

However, Romney was not alone. His assault won an endorsement from the previous GOP nominee, Arizona Sen. John McCain. It also came on a day when a group of prominent conservative national security experts warned darkly that Trump was not qualified to be president. And it coincided with a drive by Republican super PACs and other anti-Trump forces to halt the billionaire with attack ads.

A risky civil war

But unleashing a Republican civil war is fraught with risk, has staggering implications for the party itself and appears to fly in the face of the grim precedent of divided parties in American political history.

The path Romney and his allies have chosen leaves Republicans with a dilemma, assuming Trump extends his march toward the nomination in coming primaries and caucuses -- and there is little reason to think he won't.

Either they must embrace a Romney-led attempt to overturn the democratically expressed will of millions of voters and risk alienating Trump supporters who, if they reject the GOP, could effectively hand the election to the Democrats.

Or they must accept Trump as their figurehead even though embracing him would change the party itself in the eyes of moderate and conservative Republicans alike who see him as the antithesis of what they believe the GOP stands for.

JUST WATCHED

Romney speaks out against Trump's campaign (Full speech)

MUST WATCH

Romney speaks out against Trump's campaign (Full speech)17:23

Not all of Trump's policy positions are clear. But a party he leads would implicitly go into the general election as an anti-free trade, somewhat isolationist, immigration hardliner force that embraces economic nationalism and has unclear positions on social issues. That contrasts with the GOP's heritage as a pro-globalization, internationalist and even interventionist party laced with social conservatism and dedicated to a creed of small government.

The dilemma hasn't been resolved by all of the GOP's leadership, despite the full frontal attack Romney delivered. That equivocation could dilute some of the efficacy of the anti-Trump wing's efforts. And it's far from certain that Romney, who is seen by many GOP voters as a flawed candidate who botched an attempt to bounce President Barack Obama out of office, is the best messenger for such an offensive.

Party higher-ups on Thursday were sticking to the position that there could be no change to the process -- in an apparent attempt to stop the civil war from flaring out of control.

"Another day, another fascinating development," Republican National Committee Chairman Reince Priebus told CNN's Wolf Blitzer. "When we get to a nominee, this party is going to support that nominee 100%. I don't care who the nominee is. Our job is to support the person who gets the majority of delegates."

JUST WATCHED

GOP senator: I can't support Donald Trump

MUST WATCH

Not following Romney's lead

Other members of the Republican establishment in Washington -- who have more to lose than Romney because they are still in office -- were not quite so keen to rush to the barricades.

House Speaker and Romney confidant Paul Ryan declined to endorse the former GOP nominee's warning, saying only that he would speak out if he saw conservatism being "disfigured."

And Tennessee Sen. Bob Corker, the influential chairman of the Senate Foreign Relations Committee, issued a short statement pushing back at the notion that party grandees should intervene to thwart Trump.

"Here's my message to the Republican Party leaders: Focus more on listening to the American people and less on trying to stifle their voice," Corker said.

But one person who was happy to engage was Trump, who thrives on conflict and seemed to sense that he couldn't lose from a confrontation with an unsuccessful establishment nominee, lashing Romney as a "choker" who had begged him for an endorsement in 2012.

It does seems counterintuitive that in an election that has been defined by a rejection of the Republican establishment, the man to deliver a body blow to Trump would be a symbol of the GOP elite.

"I think that Romney's speech today is not going to change a single vote," said Mark McKinnon, a former strategist for President George W. Bush on CNN's "The Lead with Jake Tapper."

"If anything," he continued, "it will probably harden the support that Trump supporters have because it is just another example of the establishment all these supporters are against circling the wagons."

Whether or not they are successful or defensible, the widening anti-Trump attacks may have important electoral implications for the Republican Party. It was, after all, the spiritual leader of the party, Abraham Lincoln, who warned in another context: "A house divided against itself cannot stand."

Divided parties don't fare well

Internecine warfare has rarely made parties prosper in presidential elections.

"The short answer is it doesn't end well for that party generally," said David Karol, an political parties expert at the University of Maryland.

In recent political history, splintered parties have crashed to defeat. In 1972, Democratic nominee George McGovern was a divisive figure in his party, and he won only one state.

In 1964, another candidate that played to the extremist fringe of his party, Republican nominee Barry Goldwater, was also crushed.

Further back, in 1912, the Republican Party split over its rejection of a comeback attempt by former President Theodore Roosevelt, who decided to go it alone with his new progressive "Bull Moose Party." A Democrat, Woodrow Wilson profited from the chaos to win the White House.

"I could see Trump being a failed nominee, and the whole thing being a debacle for them with lasting consequences," said Karol. "They would probably lose the Senate, then they would lose the Supreme Court."

JUST WATCHED

Trump the unifier?

MUST WATCH

Trump the unifier?04:24

There is also precedent in U.S. history for a party to split completely. After all, the Republican Party itself emerged after such a schism in the Whig Party in the 1850s -- in that case over slavery -- though there is no similar sign or foundational issue suggesting that this crisis is an existential one for the GOP.

There is another downside from waging war with Trump: If he prevails over those who would block him, he would only emerge emboldened and would then have the capacity to change the party itself, while the party leadership would only look that much more powerless.

Still, the escalating political war may be one that the GOP has to have, given the fervent anger against a string of failed establishment nominees that has stoked the grass-roots fury that Trump has been able to exploit. In some ways, the party has been at war since at least 1994 following former Congressman Newt Gingrich's Republican revolution through to the tea party challenge to establishment forces in more recent years.

"We are going to have a really rocky year," said Heather Cox Richardson, a Boston College historian whose history of the Republican Party argues that several times in its past, the GOP has gone through a cycle of purging its more extreme elements.

"If you are really looking long term, that implies this is something that has got to be released before the party can rebuild itself in a healthy way," she said.

Richardson's advice to the party's up-and-coming leaders: "I would hide under my desk until 2020."